COLORADO RIVER DELTA, MEXICO  An unprecedented cross-border delivery of water from the Colorado River earmarked for environmental purposes is being sent to Mexico as part of a binational effort to restore some of the last remaining wetlands in this parched but biologically important region.

The delivery, which comes at a time of growing concerns about drought in the West, is part of a wide-ranging five-year pilot agreement on the Colorado River between the United States and Mexico that proponents say benefits both countries.

The water being used for the project is part of a Mexican allotment being stored at Lake Mead. Authorities with the Bureau of Reclamation have assured U.S. water users on the Colorado River, including the San Diego County Water Authority, that the delivery will not cut into their share.

The special cross-border delivery of 105,000 acre-feet — enough to supply about 200,000 households per year — is called a pulse flow. It’s being carried out by the U.S. and Mexican governments in collaboration with environmental groups.

The idea is to create flood conditions for an eight-week period that began Sunday and lasts through May 18, allowing water to flow deep into the Colorado River Delta and allow new vegetation to germinate.

The infusion is being supplemented by a lower-volume and longer-term base flow of nearly 53,000 acre-feet supplied by environmental groups through leased and purchased water rights and designed to sustain the growth.

Since the flow began, teams of scientists, environmentalists and local residents have been following the water’s flow, wondering how far down the delta it will reach — possibly as far as its original course into the Gulf of California.

Hydrologists are taking measurements, biologists are riding canoes and counting birds, and children are splashing in the river as they haven’t in years.

“I’ve been living here a long time, and I’ve never seen it with so much water,” said Edna Gamez, 37, watching Wednesday evening as her two sons joined dozens of children playing in the river beneath a bridge in the border town of San Luis Rio Colorado.

On Thursday morning, government authorities, representatives of environmental groups and farmers from the Mexicali Valley celebrated as water flowed through Mexico’s Morales Dam in the Baja California community of Algodones.

The five-year Colorado River plan was signed by the United States and Mexico in Coronado in November 2012.

It includes Mexico’s agreement to accept reduced deliveries in times of shortage, its one-time commitment to provide the United States with 124,000 acre-feet of water, as well as $21 million in U.S. funding for conservation measures and restoration efforts in the Colorado River Delta.

It also allows Mexico to store deferred Colorado water deliveries in Lake Mead — a need that first arose when the Easter 2010 earthquake ripped apart irrigation canals in the Mexicali Valley.

The water being stored for Mexico in Lake Mead benefits three lower Colorado River basin states — California, Arizona and Nevada — because cutbacks in their deliveries are triggered if the reservoir falls below a certain level.

The Colorado River agreement, called Minute 319, is an amendment to the 1944 U.S.-Mexico treaty that guarantees Mexico an annual allotment of 1.5-million acre-feet — water that up to now has been exclusively dedicated for agricultural and urban uses.

“When Mexico negotiated Minute 319, the environmental water was extremely high on the list; it was a deal breaker,” said Bart Fisher, chair of the Colorado River Board who participated in the negotiations.

The pulse is lowering the level of Lake Mead by about a foot. But Fisher and other U.S. guests at Thursday’s ceremony pointed out that the water being used for the project belongs to Mexico.

“We respect (Mexico’s) desires with regard to their sovereign water and what they want to do with it,” Fisher added.

Still, the pulse comes at a time when communities along Colorado River face increasing competition for a limited water supply. Defenders of the agreement say its emphasis on conservation and collaboration offer a new alternative for managing scarce water resources.`

“It’s not ‘How much is yours, how much is mine,’ said Karl Flessa, a conservation biologist at the University of Arizona who has studied the delta. “We’re entering an era where we have to think, ‘What’s the most efficient way to share the water, for cities, for agriculture, for the environment.”

Michael Connor, deputy U.S. interior secretary told the crowd at the Morales Dam Thursday that Minute 319 was about building trust between the United States and Mexico. “Today is about validating that trust,” he said.

He credited three environmental groups for their role in the project: Pronatura, Mexico’s largest environmental organization; the Arizona-based Sonoran Institute; and the Environmental Defense Fund.